The instant invention pertains to semiconductor device fabrication and processing and more specifically to a method of oxidizing or reoxidizing a structure.
As device dimensions continue to decrease to achieve higher density and performance, there is an increasing demand for highly conductive gate structures, interconnects, and electrodes for metal on silicon (MOS) devices. The polycrystalline silicon (xe2x80x9cpolyxe2x80x9d or xe2x80x9cpolysiliconxe2x80x9d) used for these purposes in conventional processes has several limitations, due mainly to its higher resistivity.
Recently, various metal silicides have been considered because they have a conductivity which is higher than that of polysilicon by about an order of magnitude. However, future devices with very high integration densities will, most likely, need to be fabricated with higher conductivity materials for gate structures and interconnects. Therefore, either all metal structures will need to be formed or structures which are comprised of polysilicon and a refractory metal (such as tungsten, molybdenum, cobalt and/or titanium).
With regards to the formation of a gate structure, a thermal oxidation step is needed after the gate patterning, especially for memory devices. This oxidation step is needed to remove damage caused by the patterning and etching of the gate structure (primarily at the gate edges) by reactive ion etching (RIE) and to thicken the gate oxide on the edge so as to improve its reliability and reduce sharp corners at the lower edges of the conductive gate structure. If a polysilicon and tungsten gate structure is used, this oxidation step may create problems because tungsten easily oxidizes at temperatures greater than around 350 C in an oxygen ambient.
A solution to this problem involves using H2 and H2O to oxidize any existing silicon or silicon oxide surfaces while leaving any tungsten surfaces unoxidized. See K. Nakajirna, et al., Poly-metal Gate Processxe2x80x94Ultrthin WSiN Barrier Layer Impermeable to Oxidant In-diffusion during Si Selective Oxidation, CONFERENCE PROCEEDINGS ULSI XI, 1996 MATERIALS RESEARCH SOCIETY 317-323 (1996). A problem with this approach is that a very small amount of water can be used and it is extremely difficult to controllably introduce such small amounts of water in large scale production of semiconductor devices.
In essence, the selective oxidation method of the instant invention involves the oxidation of one material (M1) without substantially oxidizing another material (M2). The chemical reactions for the oxidation of M1 and M2 are (using O2 as the oxidizer):
2M1+O2xe2x86x922M1O+E1xe2x80x83xe2x80x83(1) 
2M2+O2xe2x86x922M2O+E2xe2x80x83xe2x80x83(2) 
where E1 and E2 are the enthalpies of formation for reactions (1) and (2), respectively. Preferably, in order to achieve the selective oxidation of the instant invention, a combination of an oxidizer (preferably O2) and a reducer (preferably H2) are utilized. The minimum requirement for the successful selective oxidation of M1 while not appreciably oxidizing M2 is that reaction (1) is favored as compared to reaction (2). In other words, E1 is less than E2. In the case where M1 represents silicon, polycrystalline silicon, or amorphous silicon and M2 represents tungsten,
Si+O2xe2x86x92SiO2xe2x88x92911 kJ/molxe2x80x83xe2x80x83(3) 
(2/3)W+O2xe2x86x92(2/3)WO3xe2x88x92562 kJ/molxe2x80x83xe2x80x83(4) 
Hence, since E1 is less than E2 (xe2x88x92911 kJ/mol versusxe2x88x92562 kJ/mol), the silicon of reaction (3) will oxidize much more readily than the tungsten of reaction (4).
An embodiment of the instant invention is a method of fabricating an electrical device formed in a semiconductor substrate, the method comprising: forming an insulating layer over the semiconductor substrate; forming a silicon-containing structure on the insulating layer; forming a conductive structure on the silicon-containing structure; and oxidizing a portion of the insulating layer and the silicon-containing structure while leaving the conductive structure substantially unoxidized by introducing an oxygen-containing gas and a separate hydrogen containing gas to the insulating layer, the silicon-containing structure and the conductive structure. The electrical device is, preferably, a memory device, a capacitor, a transistor, a logic device, a digital signal processor, a microprocessor, and any combination thereof. Preferably, the oxygen-containing gas is comprised of a gas selected from the group consisting of: O2, N2O, CO2, H2O, and any combination thereof, and the hydrogen-containing gas is comprised of H2 or deuterium. The insulating layer is, preferably, comprised of silicon oxide; and the silicon-containing structure is comprised of a material consisting of single crystal silicon, doped polycrystalline silicon, undoped polycrystalline silicon, and amorphous silicon. Preferably, the conductive structure is comprised of an oxygen-sensitive material, and more preferably it is comprised of a material selected from the group consisting of: tungsten, copper, and any combination thereof.
Another embodiment of the instant invention is a method of oxidizing a first feature while leaving a second feature substantially unoxidized, the method comprised of subjecting the first and second features to an oxygen-containing gas and a separate hydrogen-containing gas. Preferably, the oxygen-containing gas is comprised of a gas selected from the group consisting of: O2, N2O, CO2, H2O, and any combination thereof, and the hydrogen-containing gas is comprised of H2. The first feature is, preferably, comprised of polycrystailine silicon, silicon oxide, or a dielectric material, and the second feature is, preferably, comprised of tungsten.
Another embodiment of the instant invention is a method of fabricating a capacitor having a dielectric between a bottom electrode and a top electrode and situated over a semiconductor substrate, the method comprising the steps of: providing the bottom electrode over the semiconductor substrate; providing a dielectric material over the bottom electrode; and subjecting the bottom electrode and the dielectric material to an oxygen-containing gas and a separate hydrogen-containing gas, wherein the dielectric material is oxidized and the bottom electrode remains substantially unoxidized. Preferably, the oxygen-containing gas is comprised of a gas selected from the group consisting of: O2, N2O, CO2, H2O, and any combination thereof, and the hydrogen-containing gas is comprised of H2. The dielectric material is, preferably, comprised of a material selected from the group consisting of: an oxide/nitride stack, BST, tantalum pentoxide, PZT, and any combination thereof.